1. Field of the Invention
The present invention relates generally to a thin film solar module and, more particularly, to the contact layers in a solar module.
2. Description of the Related Art
Thin-film solar modules made of amorphous silicon are fabricated directly on large-area substrates made of (glass or metal. To this end, an all-over contact layer (front contact) of transparent, conductive oxides (TCO) is first deposited on the substrate, above which a photoactive layer is generated which is then covered with a further, usually transparent, contact layer as the rear contact. In order to provide a suitable voltage, the thin-film solar module is subdivided into strip-shaped individual cells interconnected in series. By suitable patterning of each individual layer directly after its deposition, the layer generation and electrical interconnection form an integrated device.
The current-carrying electrodes (front and rear contacts) play a decisive part, since both their optical and their electrical properties determine the efficiency of each individual cell and the module as a whole.
The optical properties of the front contact layer, such as its transparency and light scattering properties, affect the amount of the light that reaches the photoactive layer, and thus also the photocurrent.
The electrical properties (such as contact potential and resistivity) are responsible for the discharge of the photocurrent that has been generated. The resistivity of the contact material leads to ohmic losses in the solar module.
The use of zinc oxide (ZnO) as a material for the front contacts on transparent substrates makes it possible, within certain limits, to adjust the morphology of the contact layers, which permits optimization of the quantity of the light that reaches the active semiconductor layer and an improvement in the short-circuit current of the individual cells. In order to increase the conductivity, zinc oxide layers are doped, for example with boron. If the same material is also used for the rear contact, semitransparent solar modules can be fabricated and used as the top cell in a tandem module, as a roof glazing for cars, or as glazing in the construction industry (for example as a facade module or window module).
Unprotected thin-film solar cells or modules having front and rear contacts made of boron-doped zinc oxide (ZnO:B) are found, however, not to be environmentally stable. If they have been exposed, for prolonged periods and simultaneously, to elevated temperature and humidity, this results in irreversible increases of the sheet resistance of the contact layers, especially in the externally situated rear contact. Consequently, the series resistance of the solar cell or the solar module changes, which leads directly to losses in the maximally extractable output of the module.
In known solar modules made of amorphous silicon and having front and rear contacts made of ZnO:B, the contact layers are protected against the action of humidity by being hermetically sealed. To this end there is laminated onto the rear contact, for example by means of a PVB film poly(vinyl butyral)!, a further glass sheet. To provide an outer humidity barrier, the glass package, with the solar cells situated therebetween, is coated at its rim with a polyurethane frame. For non-transparent solar cells there is applied, directly to the rear contact, as an additional water vapor barrier, an epoxy coating which at the same time serves as a residual-light reflector.
This hermetic sealing, which is required for the solar modules, of the thin layers requires a large outlay in terms of materials and labor, which leads to a considerable increase in manufacturing costs. But even with the sealing it is not possible to protect the sensitive layers completely against the penetration of moisture. The modules exhibit, in accelerated environmental tests (1000 hours at 85.degree. C. and 85 percent humidity, Qualification Test Procedure for Photovoltaic Thin Modules, Spec. No. 701, Test B13, JRC, 21020 Ispra (Va)-Italy) considerable losses, in part, in the maximum output, which can be ascribed to the penetration of water vapor into the module construction.
A contribution by S. R. Praschek et al. to the 22nd IEEE Photovoltaic Specialists Conference, Las Vegas, Nevada (USA), October 7th to 11th 1991, pages 1285 to 1289, discloses a pattern for a thin-film solar module made of amorphous silicon which is based on a glass substrate and employs front and rear contacts of zinc oxide.
EP-A-246 027 discloses a thin-film solar module of amorphous silicon which is based on a fluorocarbon film as the substrate. For the front and rear contacts, any conductive oxide is proposed, especially zinc oxide doped with hydrogen or an element of the third main group. Employed for the rearside covering are films of polyester, Teflon.TM. or Tedlar.TM..
EP-A-204 562 discloses a transparent thin-film solar module which is proposed for use as a facade element for a building or as the roof for a vehicle. The module is based on a glass substrate, has an indium tin oxide rear electrode and a rear covering made of a further glass sheet.
In a contribution by R. H. Mauch and H. W. Schock to the 10th E. C. Photovoltaic Solar Energy Conference, Lisbon, Portugal, April 8th to 12th 1991, pages 88 to 91, aluminum-doped zinc oxide is proposed for use as the window layer for solar modules.
U.S. Pat. No. 4,316,049 discloses a module, interconnected in series, composed of individual tandem solar cells made of amorphous silicon. Interconnection of the individual sells is effected via metallic rear contacts.